The present invention relates to a method of protecting equipment intended to operate at high temperature in the presence of a fluid containing at least one hydrocarbon and/or carbon monoxide against corrosion by metal dusting, in which pieces of the equipment are made from alloys containing nickel, iron, chromium and/or aluminum, and in which said pieces of the equipment are protected from said corrosion by a protective coating.
In the rest of the description, the term “high temperature” means a temperature at least equal to the temperature at which the metal dusting phenomenon appears. It is commonly understood that this phenomenon appears at temperatures of about 450° C. or more.
Corrosion of the metal dusting type is a major problem in industrial processes for which the alloys used for the production of equipment items and of pipework operate at high temperature in the presence of hydrocarbons and/or carbon monoxide. It has been found that, after a relatively long period of operation, a corrosive attack is manifested by the gradual dusting of the alloy.
In particular, in the case of steam reforming, natural gas or other hydrocarbons are transformed into carbon monoxide, hydrogen and carbon dioxide by catalytic reaction over nickel catalysts. Downstream of the steam reforming furnace, the equipment is subjected to corrosion by metal dusting, which damages the alloys from which it is constructed.
Various solutions have been proposed in order to reduce this phenomenon, including the use of surface coatings which make it possible to strengthen the corrosion resistance of the alloys. The aluminization of the surfaces intended to be subject to this corrosion is in particular used in order to limit this phenomenon.
To optimize this protection, EP 0 903 424 describes a method intended to improve the protection of a so-called high-temperature alloy containing iron, nickel or chromium from metal dusting, a method consisting in depositing a thin layer of metal from group IV or V of the periodic table with a thickness of between 0.01 and 10 μm on the surface to be protected, and to rebake the treated surface in an inert atmosphere.
However, although the alloy coatings provide satisfactory protection, there is still the problem of connections between protected pieces. Bonding by welding between protected pieces requires using particular connections in order to provide continuity of protection.
At present, no satisfactory solution for this problem of joining protected pieces is known.
This is because, when the pieces to be connected are protected before welding, the welding destroys the protection of the welded region and adversely affects the protection of the adjacent region. When this protection is applied after welding, the protection is then produced over a filler material with a different composition and/or structure; in this case, the diffusion speeds of the various chemical elements are affected leading to different and often reduced quality and thickness in the welded region with respect to the base material unaffected by the welding. Furthermore, given the available welding material, the piece to be welded to the support cannot be fastened for small-diameter pipework. It is therefore not possible to protect junctions directly on site.
Thus, whatever the improvements in protection provided on the type of alloy used for producing the equipment, on the type of surface coating used for protecting the alloy and on the operating conditions of the industrial process for which the equipment is intended, it is still the case that the particular problem of corrosion at the connections between protected pieces remains. Now this corrosion, even when localized, may lead to the failure of equipment items which operate under pressure conditions which may reach 50 bar.